Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, lithium secondary batteries having high energy density and voltage are commercially available and widely used. The lithium secondary batteries generally use a lithium transition metal oxide as a cathode active material and a graphite-based material as an anode active material.
However, the anode formed of the graphite-based material has a maximum theoretical capacity of only 372 mAh/g (844 mAh/cc), thus suffering from a limited increase of capacity thereof. Consequently, such a graphite-based anode is incapable of carrying out a sufficient role as an energy source for next-generation mobile equipment undergoing rapid development and advancement. Further, lithium metals, studied for use as the anode material, have a very high energy density and thus may realize a high capacity, but raise problems associated with safety concerns due to growth of dendrites and a shortened cycle life as charge/discharge cycles are repeated. In addition, use of carbon nanotubes (CNTs) has been attempted as an anode active material, but various problems have been pointed out such as low productivity, expensiveness and low initial efficiency of less than 50%.
In this connection, a number of studies and suggestions have been recently proposed as to silicon, tin or alloys thereof, as they are known to be capable of performing reversible absorption (intercalation) and desorption (deintercalation) of large amounts of lithium ions through the chemical reaction with lithium. For example, silicon (Si) has a maximum theoretical capacity of about 4020 mAh/g (9800 mAh/cc, a specific gravity of 2.23) which is substantially greater than the graphite-based materials, and thereby is promising as a high-capacity anode material.
However, upon performing charge/discharge processes, silicon, tin or alloys thereof react with lithium, thus undergoing significant changes of volume, i.e., ranging from 200 to 300%, and therefore repeated charge/discharge may result in separation of the anode active material from the current collector, or significant physicochemical changes at contact interfaces between the anode active materials, which are accompanied by increased resistance. Therefore, as charge/discharge cycles are repeated, the battery capacity sharply drops, thus suffering from a shortened cycle life thereof. For these problems, when a conventional binder for a graphite-based anode active material, i.e., polyvinylidene fluoride or styrene butadiene rubber, without any special treatment or processing, is directly applied to a silicon-or tin-based anode active material, it is impossible to achieve desired effects. In addition, when an excessive amount of a polymer as a binder is used to decrease volumetric changes occurring during charge/discharge cycles, separation of the active material from the current collector may be decreased to some degree, but the electrical resistance of the anode is increased by an electrical insulating polymer used as the binder and the amount of the active material is relatively decreased, which consequently results in problems associated with a reduced battery capacity.
In order to cope with such problems, there is an urgent need for the development of an excellent binder which is capable of maintaining the adhesion with the current collector while absorbing large volumetric changes of anode active materials occurring during the charge/discharge process in lithium secondary batteries using the silicon-or tin-based anode active material.
On the other hand, use of polyvinyl alcohol or thermosetting plasticized polyvinyl alcohol having superior adhesive strength has been attempted as a binder for an electrode of a lithium secondary battery (see Japanese Unexamined Patent Publication Nos. 1999-67216, 2003-109596 and 2004-134208). However, the above-mentioned polyvinyl alcohol binder exhibits superior adhesive strength, as compared to conventional binders, but suffers from a very low viscosity, non-uniform application of the binder on copper foil as a current collector and process problems associated with thermal treatment necessary to improve adhesion between the electrode mix and the current collector.
For example, International Publication No. WO 2000-007253 discloses a binder which is comprised of a combination of polymers selected from a variety of polymers including polyvinyl alcohol and polyurethane. Korean Patent Application Publication No. 2006-001719 A1 discloses a method of preparing an anode active material for a lithium secondary battery, using a resin composite which is prepared by coating graphite and at least one metal selected from the group consisting of Si, Sn and A1 with a fixing agent such as a polyvinyl alcohol resin, a urethane resin, or the like. In connection with the preparation of the cathode for a lithium-sulfur secondary battery, Korean Patent Application Publication No. 2003-0032364 A1 discloses a technique which involves preparation of an active material including a conductive material by binding between the sulfur active material and the conductive material using a combination of polymers selected from a variety of polymers including polyvinyl alcohol and polyurethane as a first binder, and employs a second binder component insoluble in the solvent of the first binder, as a binder between the active materials including the conductive material and between the active material and the current collector.
However, the aforementioned conventional arts merely exemplify various kinds of polymers that may be used as binders or fixing agents, and do not suggest a mixture binder made of a certain combination in accordance with the present invention, as will be illustrated hereinafter. Further, none of the aforementioned prior arts teach or imply that the selection of such a certain combination will bring about significant synergistic effects in adhesive strength and elongation percentage of the binder.
Meanwhile, Korean Patent Application Publication No. 2002-062193 A1 discloses a semi-interpenetrating polymer network (semi-IPN) formed by the combination of a polyvinyl alcohol derivative and a compound having crosslinkable functional group(s), as a binder, wherein the polyvinyl alcohol derivative is a polymer compound having an oxyalkylene chain-containing polyvinyl alcohol unit in which hydroxyl groups are partially or completely substituted by oxyalkylene-containing groups, and compounds having two or more reactive double bonds are exemplified as the compound having crosslinkable functional group(s). However, this art suffers from a serious problem in that partial or complete substitution of hydroxyl groups of polyvinyl alcohol leads to significant deterioration in physical properties of the polyvinyl alcohol, thus failing to express a desired level of adhesive strength.